Enhanced removal of trivalent chromium from leather wastewater using engineered bacteria immobilized on magnetic pellets
Graphical abstract
Introduction
Chromium is one of the most commonly used heavy metals in industries involving pigments, metal plating, and leather (Zewdu et al., 2018). Chromium exists mainly in two forms: trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) (Oliveira, 2012). Mostly, the studies focus on Cr(VI) containing wastewater treatment using different methods like Chitosan magnetite composite material and polyaniline (Gu et al., 2018a; Gu et al., 2018b). However, Cr(III) is more stable compared to Cr(VI) and plays an important role in the leather process (Apte et al., 2006). Only 60–70% of the chromium salts were utilized for the tanned hides, while 30–40% were discarded with solid and liquid as contaminants during the leather process (Esmaeili et al., 2005). The global leather industry currently produces >300 million tons wastewater and 64,320 tons sludge every year (Isarain-Chávez et al., 2014). These waste products are the source of substantial environmental pollution and even probably threaten human health owing to their capacity to promote both allergic skin reactions and various forms of cancer (Lukina et al., 2016). Several traditional methods, including chemical precipitation, ion exchange, and electric coagulation, have been evaluated in an attempt to remove Cr(III) from leather wastewater (Fahim et al., 2006; Mella et al., 2015; Nur-E-Alam et al., 2020). Although the treatment efficiencies were overall quite high, there are limitations to these procedures such as the addition of chemical reagents is easily to cause secondary pollution; difficulty to separate adsorbents from wastewater system; membrane fouling, and high cost for electric coagulation (Chen et al., 2016).
Recently, bioremediation mediated by microorganisms have gained more attention in facilitating heavy metals removal with it being environment friendly and having relatively low-cost (Giller et al., 1998). Microorganisms can eliminate heavy metal contamination via mechanisms involving biodegradation and/or adsorption (Parameswari et al., 2009). Previous study reported that several strains such as Geobacillus thermodenitrificans, Pseudomonas putida, and Bacillus cereus have the capacity to adsorb various heavy metals, including Cd, Pb, and Cu (Chatterjee and Ray, 2008; Chatterjee et al., 2010; Pardo et al., 2003). Furthermore, some other microorganisms can also promote detoxification of heavy metals, for example, the reduction of Cr(VI) to Cr(III) via redox reactions (Wu et al., 2015). However, there are a few proteins that are capable of adsorbing Cr(III) (Kotrba et al., 1999). A better understanding of this process is needed to develop microorganisms with specific proteins capable of adsorbing Cr(III).
Microorganisms can be immobilized via attachment to carriers that can facilitate the introduction of biologically active microbes at high local concentration and provide substantial advantages with respect to removal of contaminants (Bouabidi et al. 2019; Wang et al., 2013). Previous studies reported that featured microbes immobilized on polyvinyl alcohol (PVA) can be used for removal of Cd(II) and 2,4–dichlorophenol (DCP) at a high efficiency of 78% and 95.4%, respectively (Huang et al., 2015). However, it was difficult to remove the PVA pellet carriers after operation, which easily resulted in the second pollution. Recently, microbial immobilization on magnetic nanoparticles has been explored as a promising method to address the issue that a magnetic field can be applied to facilitate recycle after adsorption (Tural et al., 2017). Magnetic nanoparticles have been studied and applied in many fields such as in the medical field for the targeted treatment of liver fibrosis, in biomedicine for the focus on imaging and in the environment for forming compounds to remove contaminants (Beljaars et al., 2002; Ding, 2019; Sahu et al., 2019). In addition, immobilization on magnetic nanoparticles is also contributed to increasing the mechanical strength of adsorbent material (Tang and Lo, 2013).
MerP is a cysteine-rich protein that contains two amino acid separated by cysteine residues; the tertiary structure of this region forms a site for heavy metal selection and binding (Kao et al., 2008). MerP on the bacterial cell surface is capable of adsorbing Cr(III) (Huang et al., 2003). Meanwhile, an engineered strain of Escherichia coli that overexpresses MerP may be effective in removing Cr(III) by adsorbing it from leather wastewater. In addition, immobilization can also enhance its adsorption efficiency (Tang and Lo, 2013). In this study, the E. coli strain M-BL21 was immobilized on a magnetic carrier to facilitate its use for Cr(III) adsorption and recovery (Bouabidi et al., 2018). The internal and external structures of the magnetic pellets were evaluated by scanning electron microscopy (SEM); their magnetic properties and reusability were also evaluated. Adsorption of Cr(III) by magnetic pellets conformed to pseudo-second-order kinetic and Freundlich models. Magnetic pellets were used to treat leather wastewater in real time, with 88.2% adsorption efficiency. Therefore, our findings suggest a clean and economical method for leather wastewater treatment with broad development potential.
Section snippets
Reagents and materials
Biological reagents used for the extraction and cultivation of microorganisms were purchased from Tiangen Biotech Co., Ltd (Beijing, China). Cr(OH)m (SO4)n·2H2O was purchased from Hong Liang leather factory located in the Gansu province. Fe3O4 was purchased from Suzhou Youzirconium Nanomaterial Co. Ltd. Analytical reagent grade chemicals used in this study were provided by Alighting Reagent Co., Ltd (Shanghai, China). All of the chemicals were of analytical grade. All the solutions were
Adsorption of Cr(III) by E. coli strain M-BL21
We evaluated the capacity of the E. coli strain M-BL21 to adsorb Cr(III) presented at various concentrations (Fig. 1). Escherichia coli M-BL21 and E. coli M-N-BL21 adsorbed Cr(III) presented at a concentration of 70 mg/L at a maximum of 61.4% ± 1.3% and 51.1% ± 0.9%, respectively; the control P-BL21 was capable of Cr(III) adsorption only up to 27.7% ± 1.1%. These results revealed that the adsorption efficiency of strain M-BL21 increased by 10.3% compared to that of M-N-BL21. The main reason was
Conclusions
Genetically engineered strains of E. coli with cell-surface display of the cysteine-rich protein MerP (strain M-BL21) were capable of enhanced adsorption of Cr(III) at 2.38 mmol/g cell. Escherichia coli-derivatized magnetic pellets were prepared by immobilizing the recombinant strain M-BL21 on magnetic carriers. The removal efficiency for Cr(III) reached >90%, with the fraction of SA at 2.5%, PVA at 8%, Fe3O4 nanoparticles at 3.5%, and the strain M-BL21 at 3 g/L. The adsorption kinetics was
CRediT authorship contribution statement
Jicun Wang: Conceptualization, Data curation, Investigation. Shuai Zhao: Methodology, Writing – original draft. Zhenming Ling: Writing – review & editing. Tuoyu Zhou: Formal analysis, Software. Pu Liu: Visualization, Writing – review & editing. Xiangkai Li: Conceptualization, Funding acquisition, Project administration, Resources, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by National Natural Science Foundation of China Grant (No: 31870082), Gansu Province Major Science and Technology Projects (No: 17ZD2WA017), Central Universities in China Grant (grant number lzujbky-2020-83) and Chengguan District Science and Technology Development Project (2020JSCX0019).
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These authors contributed equally to this work.